This invention relates to optical disk data storage systems and more particularly to a servo system which includes both fine and coarse access and tracking systems.
Optical data storage systems which utilize a disk to optically store information have been the object of extensive research. Like magnetic disk units, these optical disk storage units must have a servo system which controls the positioning of the read/write head to provide direct access to a given track of data recorded thereon, and to accurately follow this track while it is being read or when it is initially written onto the storage medium. To date, most of the prior art optical storage systems have had one of three types of servo systems: physical groove, external encoder or optical feedback.
The simpliest of the systems is the physical groove as is shown, for example, in U.S. Pat. Ns. 4,260,858 and 3,654,401. In such systems, the optical disks are provided with physical grooves, either in a spiral or circular pattern, and an optical read/write head is provided with a stylus or other physical means of engaging the groove. There are several drawbacks associated with this type of system. First, wear is a significant factor. Typically, the disks are formed using dies or molds, which dies or molds are subject to wear during the manufacturing process, hence necessitating their replacement and creating tolerance problems in the formed disks. Physical contact with the head guide stylus during use also causes disk wear, introducing noise. Further, rapid access involving radial movement of the head is difficult to accomplish.
One known approach to overcome the problems associated with the physical groove systems is to dedicate an entire data disk surface to servo tracks. This approach has worked well in magnetic disk systems, where a plurality of magnetic disks are usually provided in a stacked disk pack with a common spindle. The use of one disk surface for servo tracks does not seriously detract from the data storage capacity of such a system. Optical disk systems, on the other hand, in order to be suitable for use in a commercial environment, desirably have only one disk on a spindle with at most two surfaces available for recording both the data and servo information. It is not feasible, therefore, to dedicate an entire optical disk surface to servo tracks without severely sacrificing data storage capacity.
While magnetic disk servo systems can be adapted for use with optical disks, this approach is also very inefficient. Data track density can be made much higher in optical recording systems than in magnetic disk systems. Optical systems are capable of recording in an extremely narrow data track approaching one micron in width. This allows an increase in track densities on the order of 15 times the densities used in state of the art magnetic disk systems. An extremely accurate and sensitive servo system must be used to position the read/write head over such a track.
The optical disk systems that have heretofore provided the highest capability have employed optical feedback for tracking. Changes in reflected or transmitted illumination received from the disk are monitored by appropriate equipment. Illumination changes indicate the occurance and direction of an off-track condition. Appropriate circuitry senses the change and activates a galvanometer controlled mirror in the light path steering the light beam(s) in the appropriate direction to continue track following. Such tracking systems can be extremely accurate and responsive but have range limitations on the order of 100 microns. This limitation arises essentially from the optics through which the light travels between the mirror and the disk. Galvanometer mirror systems allow rapid random access within this range but the optical head must be moved across the disk to obtain access to wider areas. Modern data storage applications require fairly rapid access to any data storage area on the disk, and thus require accurate track accessing over a range of many centimeters and accurate track following upon access.
Galvanometer type servo positioning systems typically access other tracks in one of three ways:
(1) after accessing a first track (the starting point), successively adjacent tracks are accessed, and identified, one at a time, until the desired track is reached;
(2) after identifying the track presently accessed (the starting track), and the track to be accessed (the target track), a determination is made as to the number of tracks n between the starting track and the target track, and whereupon access is achieved by moving the head n tracks, as determined by counting the individual tracks between the starting and target tracks; locking onto each track succesively as the count progresses; or
(3) after identifying the starting and target tracks as in method (2) above, a velocity servo is enabled which achieves access by forcing the galvanometer to follow a prescribed velocity profile that steers the optical beam to the vicinity of the target track, whereupon the track identification is read to verify that the desired target track has been reached.
Access method (1) above is extremely slow. Method (2), on the other hand, provides faster access and can be realized with relatively simple counting circuitry. Method (3) provides the fastest access, but also requires the most complex circuitry for its realization. All three methods, however, are limited to the range of the galvanometer system, and do not, therefore, provide the needed rapid random access to all portions of the disk.
It is also a desirable feature in commercial optical disk systems to provide a removable/replacable disk. This allows disks to be readily changed so that information recorded on different disks can be readily accessed. In a removable/replacement disk system, gross errors in alignment (up to several hundreds of microns) of the disk with respect to the head are unavoidable. Such alignment errors will typically exceed the 100 micron tracking radius of most galvanometer systems. It is therefore necessary to provide a servo system which will compensate for these gross errors and which will reliably position the read/write head with direct access over a large area of the optical disk.
Various systems have been developed to improve random access, or compensate for gross positional errors, or both. For example, U.S. Pat. No. 4,094,010 utilizes a plurality of fixed read/write heads spanning an entire disk surface. Each head is associated with a single servo track and a band of data tracks. While rapid access is assured by such a system, the plethora of tracking heads and ancillary equipment required greatly increases the cost and complexity of the system. The optical systems of U.S. Pat. Nos. 4,275,275, 4,160,270 and 4,282,598 each develop a coarse tracking error signal for use by a coarse positioning system to control head movement during tracking. The coarse track signal is developed from the tracking error signal generated by the galvanometer fine tracking system. In U.S. Pat. No. 4,037,252 a coarse control signal is generated from the movement of the fine tracking galvanometer mirror itself as opposed to the signal developed from illumination data obtained from the disk. A significant drawback of these coarse positioning systems is that they do not decouple fine tracking errors from coarse tracking errors, thereby providing a less stable system. Moreover, these systems provide no improved direct random access capability.